Chilean soils degradation is a serious problem affecting directly the productivity of most of the crops. Some evidence indicates sewage sludge can improve soil physical properties, especially when combined with sawdust. The objective of this work was to evaluate the effect of different mixtures of sewage sludge (LU) and sawdust (RA) on water-stable aggregates and available water content of a degraded Alfisol cropped with hybrid ryegrass (Lolium × hybridum Hausskn.), under greenhouse conditions. The experimental design was a completely randomized with a factorial arrangement of 2 × 9 (ryegrass × rates of sludge and sawdust). Treatments were 25 and 50 Mg LU ha -1 mixed with 10, 25, and 50 Mg RA ha -1 . Ratio mixtures of LU/RA were set in pots with hybrid ryegrass. Soil available water content (AW), percentage of macroaggregates (MA) and water-stable aggregates (AWD) were evaluated. Ryegrass aboveground biomass was also measured. Mixtures of LU/RA amended to the soil increased MA and AWD, an effect that was most evident at 50 Mg ha -1 . The AW value was greater when LU was amended to 50 Mg ha -1 mixed with RA at doses of 10, 25, and 50 Mg ha -1. There was greater aboveground biomass of hybrid ryegrass, directly related to increasing doses of LU and RA in the mixtures. The addition to soil of both organic residues combined, LU and RA, improved the physical parameters studied, especially in the presence of hybrid ryegrass.

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There are few studies about the impact of sewage sludge on the biological properties in Alfisols of the Chilean Coastal Range drylands. Hence, the objective of this study was to evaluate its effect on the microbial respiration and enzymatic activities of a degraded Alfisol located in the Bío Bío Region (Chile) that was cropped with yellow serradela (Ornithopus compressus L.). Sludge was added to the soil at rates of 15, 30, and 60 t ha -1 ; he following treatments were defined: L15-P = 15 t ha -1 sludge + O. compressus; L30-P = 30 t ha -1 sludge + O. compressus; L60-P = 60 t ha -1 sludge + O. compressus; L15 = 15 t ha -1 sludge; L30 = 30 t ha -1 sludge; L60 = 60 t ha -1 sludge; CP = non-amended soil, cropped; and C = non-amended soil, no crop. Soil microorganism activity was evaluated by respirometry. Hydrolytic enzyme activity representative of soil C, N, and P cycles was determined. Crop phytomass development was also evaluated. The amount of C-CO2 produced by soil microorganisms was directly proportional to the dose of amended sludge (p ≤ 0.05). Similarly, greater β-glucosidase, urease, and acid phosphatase were more active at 60 t sludge ha -1 . However, both respiratory and enzymatic activities were greater (p ≤ 0.05) in treatments with sludge-amended soil cropped with O. compressus. This greater activity was notorious when the legumes achieved greater phytomass development, thus highlighting the root's stimulating effect on soil biological activity.

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